1. Field of the Invention
The present invention relates to an apparatus for Plasma Chemical Vapor Deposition (hereinafter referred to as Plasma CVD).
2. Description of the Prior Art
The Plasma CVD has recently attracted attention as a process for forming a film by deposition. The process comprises highly reducing a pressure in a reaction chamber, introducing starting gases into a reaction chamber and decomposing said gases by glow discharge upon arc discharge to form a deposited film on a substrate which is placed in the reaction chamber.
A film of an amorphous silicon (hereinafter referred to as a-Si), which is produced from a silane (SiH.sub.4) gas as a starting gas, has relatively less localized energy level in a forbidden band of a-Si, thereby enabling one to control electronegativity by doping an impurity of the substitutional type. Thus, an a-Si may be used as an electrophoto-sensitizing member.
FIG. 1 shows a Plasma CVD apparatus to form a layer of a-Si on a cylindrical substrate 3, and more particularly, a cross-sectional view of typical so-called batch type Plasma CVD apparatus which comprises a single vacuum chamber 1 and 10.
A rotating means 5 which driven by motor M is provided at a center of a vacuum chamber body 10 at its base. The cylindrical substrate 3 is placed on and fastened to the rotating means 5 and heated with a heater 4 provided therein. The heating is necessary for maintaining the substrate at a higher temperature, for example, at 250.degree. C., in order to form a stable a-Si layer on the substrate 3. A cylindrical shaped cathode electrode 2 connecting to a high frequency power source 7 is positioned around the substrate, and is protected with a cylindrical shield 6 in order to confine a plasma within the shield. A gas supply unit 8 has an inlet at the bottom of the shield 6 so as to introduce a starting gas, such as silane gas, into the reaction chamber. An evacuating unit 9 for evacuating the reaction chamber has an opening at the bottom of the vacuum chamber body 10.
According to the apparatus of the present invention, after heating the substrate 3 and upwardly introducing the starting gas into the reaction furnace (within the shield 6) from the bottom, radio frequency power is applied to the cathode electrode 2 from the power source 7 thereby creating a plasma discharge between electrode 2 and the substrate 3 which is used as an anode electrode. The starting silane gas is decomposed to form a-Si film on the substrate 3. The remaining gas, which is not reacted, is removed by holes 6a provided at upper part of the shield 6, passes downwardly through a space between the outer surface of shield 6 and the inner surface of a vacuum chamber body 10, and is removed from the apparatus through the evacuating unit.
On the other hand, according to the prior known Plasma CVD apparatus, there is the drawback such that since a reaction apparatus may be used for forming film on the substrate, singly in each process, it is poor in efficiency as an industrial production system, particularly in mass production. Furthermore, since the plasma reaction takes place within the area in the reaction furnace surrounded by the shield, the deposited film is formed on a whole exposed surface area in the reaction furnace, that is, not only on the substrate surface, but also on the surface of a high frequency electrode (cathode) as well as inner surface of the shield. Accordingly, the ratio of an amount of deposited film to the introduced starting gas is so small that the wasted portion of the starting material may be larger. Additionally, a deposited a-Si film both on the cathode electrode surface and inner surface of the shield often peels off and attaches on the substrate as a contaminant, thereby causing a defect. Furthermore, since the inlet for introducing the starting gas or the outlet for evacuating the remaining gas is to be provided on the top or bottom face of the vacuum chamber due to a constructional requirement, the film deposited on the substrate forms unevenly according to the direction of a gas flow.